Implantable medical devices for remedial treatment of and compensation for cardiac, neural and muscular disorders are known in the art. These devices range from cardiac pacemakers as described in U.S. Pat. No. 4,712,555 to Thornander et al., to microstimulators as described in U.S. Pat. No. 6,208,894 to Schulman et al. The quest for minimization of such devices, especially in the area of microdevices such as microstimulators and microsensors continues. Paramount in this quest has been the challenge of efficiently providing a reliable and stable power source to power the device or charge its internal battery. The quest has further addressed the communication medium to facilitate information, data and command signal transfer and exchange between the microdevice and a corresponding microdevice control unit.
Heretofore, wireless communication between the control unit and the implanted device including microdevices, has been described as being implemented by means of a modulated signal, such as a time varying (alternating current AC) magnetic field or light source. In certain instances, wireless communication is also intended as a means of power delivery to an implanted microdevice. This may be achieved by way of a time varying magnetic field generated by an inductor positioned in proximity to the microdevice. The inductor may be formed on a flexible support which contains a series of closely wound electrically conductive wires. When these wires are energized, a magnetic field is generated in the vicinity of the wires.
Such flexible arrangements are useful when it is necessary to bring the inductor in close proximity with microdevices that are implanted in regions of the body characterized as being very contoured. For example, when microdevices are implanted on either side of a patient's neck, the inductor, and therefore the inductor support, must be sufficiently flexible and pliable to permit the inductor to be wrapped around the patient's neck so that the inductor will be in close proximity to the implanted microdevices. With the inductor being so positioned, a maximum magnetic coupling is achieved between the inductor and microdevice. This enables communication, whether it is intended, for example, for data transfer or charging a rechargeable microdevice battery, to be efficiently and reliably realized.
Data transmission between such devices may involve the use of magnetic field modulation techniques using known data transmission protocols. Good wireless communication with magnetic field coupling is best realized when the magnetic field strength is unaffected by manipulation of the inductor support as well as the introduction of magnetic field altering implements in the vicinity of the inductor. Unfortunately, such ideal circumstances are generally not possible and the effective inductance value of the inductor often is caused to change. This can occur, for example, if the inductor is bent or distorted when applied to fit the contour of a desired location of a body.
Magnetic field coupling systems generally use a power source that drives a tuned circuit. The tuned circuit generally comprises an inductor and a capacitor. The maximum power delivered to the tuned circuit, and therefore the maximum magnetic field strength produced by the inductor, occurs when the resonant frequency of the tuned circuit matches a reference frequency, such as a driving frequency.
Most often, the driving frequency of the power source relates to the nominal values of the capacitor and the inductor. Changes in inductance value of the inductor may have a severe impact on the resonant frequency of the tuned circuit. This results in a correspondingly negative effect on the magnetic field generated by the inductor. Deterioration of the magnet c field strength would comprise communication integrity or power transfer between the control unit and the relevant microdevice.
Inductor shape changes from circular to a flattened oval can result in a reduction of inductance value of as much as 50%. Such inductance value changes may be dynamic in nature. Hence, there is a need for a tuning system that dynamically and adaptively adjusts the resonant frequency of the tuned circuit and maintains the resonant frequency substantially equal to the driving frequency. The present invention addresses this and other issues.